Today, as people increasingly use more handheld apparatuses (e.g., mobile phones, laptop computers, DVD/CD/MP3 players, PDA, etc.), compactness and lightweight thereof is essential. Accordingly, the first challenge is to produce compact and thin semiconductor packages used in the handheld apparatuses. For conventional semiconductors, employed is a surface mount packaging manner (gull-wing SO format or QFP (quad-flat-package)) in which a semiconductor is connected to a circuit board via an extended lead, which does not meet all of the current requirements aforementioned. In particular, for a handheld communication apparatus in which a high frequency of several GHz is used, performance and efficiency thereof is degraded due to exothermic reaction by means of dielectric loss of semiconductor devices.
Recently, quad flat no-lead packages (QFN) without leads to address such requirements of semiconductors are highly demanded. In QFN's, the lead is not extended but exposed toward a package bottom as a land form around a die, to be soldered directly onto a circuit board. It is thus possible to produce a package which is significantly smaller and thinner than a package with a lead and which advantageously occupies 40% less area than an existing package on a circuit board. In terms of heat emission, a lead frame is positioned on the bottom of a package and a die pad is exposed directly toward the outside to be advantageous in heat radiation, differently from a typical manner that a lead frame to place chips thereon is covered by sealing resin. Accordingly, the present package is superior in electrical properties to a conventional package with an extended lead, and has self-inductance half of that of the typical package.
Since there exists an interface where the lead frame and the sealing resin surface coexist on the bottom of the package as such, the sealing resin easily goes between the lead frame and a general molding frame to contaminate the land surface or the die pad surface if the general metal frame is used. Therefore, it is essential to use an adhesive tape to laminate a lead frame, subsequently to undergo the QFN manufacturing process and the resin sealing process in order to avoid bleed-out or flashing of the sealing resin during the resin application process.
Meanwhile, the process which involves using a heat-resistant adhesive tape while manufacturing a semiconductor device generally consists of tape lamination→die attach→wire bonding→EMC molding→detaping.
Firstly, in the process of tape lamination, an adhesive tape is adhered to a copper or PPF (Pre-Plated Frame) lead frame with a laminator. In this case, the properties of the adhesive tape depend on the type and scheme of the laminator. The exemplary scheme of tape lamination includes using a roller, hot press, a combination thereof, and pressing only the dam bar on a lead frame, etc. It is essential that the adhesive layer closely laminates the lead frame without any bubbles and adhesiveness should be kept without delamination while handling the lead frame laminated with the adhesive tape.
Secondly, in the process of attaching a die to a lead frame, an adhesive layer of epoxy, polyimide or silicon in a paste phase or a film shape is cured for a period of 20 to 60 minutes at 150 to 170° C. to attach the die to the lead frame simultaneously. During this process, the adhesive tape used to laminate the lead frame should not adversely affect die attachment while keeping the adhesive layer or the base layer without chemical and physical deformation and with stale heat resistance.
Thirdly, in the wire bonding process, the die is connected electrically to the land of lead frame by means of gold, aluminum, copper wire, etc. In this process, the wire is bonded by means of heat, pressure or ultrasonic waves. This process continues for a minimum of 20 minutes to a maximum of 2 hours or so at 100 to 300° C. For secure bonding while the wire is bonded to the die and the land, respectively, high temperature, pressure or even ultrasonic vibration is required. Therefore, if there occur changes in physical properties of the tape which supports the lead frame on the bottom thereof in the wire bonding process, it may result in poor wire bonding, which wire is the most important component in a semiconductor device. In other words, the wire may be damaged or it may create a poor bonding interface. Accordingly, it is essential that the adhesive layer itself including the substrate has good heat resistance, deforms little against external physical force and also has good durability.
Fourthly, in the EMC molding process, the lead frame/tape which went through the die attach process and the wire bonding process is sealed in a molding frame with sealing resin. This process is carried out for 3 to 5 minutes at a high temperature of 175 to 190° C. While each element or a plurality of elements is sealed simultaneously, in order to prevent the resin from flowing into and penetrating into the interface between the lead frame and the tape, the tape should be adhered closely to the lead frame at a high temperature. Otherwise, the flow at high temperature and high pressure may result in bleed-out or flash of the sealing resin. In addition, the adhesive layer directly contacting the sealing resin may react with the sealing resin to leave residual adhesive on the sealing resin surface in a later process. In other cases, sealing resin flow may cause physical shear in the adhesive layer to result in an uneven sealing resin surface.
Fifthly, in the detaping process, as mentioned in the EMC molding process, it is essential that the sealing resin surface that contacted the adhesive layer should be physically or chemically uniform and there is no residual adhesive left, after removing the tape. There should be no adhesive left on the surface of lead frame as well. Detaping may be carried out manually at room temperature or by means of an automated machine which can achieve heating. In case of using a machine, the tape-adhered lead frame which went through the EMC molding process with sealing resin is made to pass through an oven or hot plate for detaping at a heated state of proper temperature.
To sum up for features required for an adhesive tape used in the process of manufacturing semiconductor devices as mentioned above, the tape should be adhered closely to the lead frame without apparent bubbles formed, in conformity with the scheme of lamination by each laminator, and the tape should not experience physical or chemical changes in the temperature range and for the process time required for good die attachment or wire bonding. In other words, it should be avoided that a part of the adhesive layer comes out in an outgas phase to be absorbed into the surface of a semiconductor device element and in turn to degrade interface coupling force that should be bonded. Also, changes in the dimension of the tape or extreme changes in the values, e.g., the modulus of elasticity or viscosity which is a physical property of the adhesive layer to cause the adhesive layer to flow down or to be crumbled due to too high hardness may have an adverse effect on the reliability of the semiconductor device. During the process of EMC molding, the tape should be adhered closely to the lead frame so that the sealing resin does not penetrate into the interface and thus not to contaminate the surface of the lead frame. Of course, it is possible to apply a deflash process after the detaping process, but it is recommended that there is no additional process of washing in terms of efficiency and economy. There should be no peculiar reaction between the adhesive and the sealing resin so that the state of the sealing resin surface is implemented as the sealing resin was originally intended, and there should be no adhesive left on the surface. Also, there should be no adhesive residue left on the surface of lead frame.
In particular, in the present invention, it is a further object to develop an adhesive layer which meets required characteristics of a tape and which is suitable for use with a laminator for lamination by means of a hot press selected among the various manners of lamination, as described above. In this case, the adhesive layer should not exhibit adhesiveness to materials, e.g., stainless steel (STS), which is a general material for laminator components, at room temperature, but implement adhesiveness only when a given heat and pressure is applied thereby to enable the layer to laminate the lead frame.
There is a limitation to implement such temperature-dependent initial adhesiveness with conventional acrylic or silicon based adhesives. In other words, it is impossible to implement near zero adhesiveness at room temperature but full adhesiveness only in heating. It might be possible to minimize physical coupling force of a tape applied to an adhered object by means of adjustment of adhesive components or functional groups. However, the adhesive is closely attached to an adhered object due to the viscosity of the adhesive via a basically low glass transition temperature, without great external pressure, thereby to adversely affect the lamination process.
With the heat-curable acrylic adhesive, adhesive residues are left on the lead frame or on the sealing resin surface after detaping because of a limitation in intrinsic heat-resistant cohesion or good tack. It is well known in the art that, with a silicon adhesive, if the non-cured adhesive of low molecular weight is evaporated in an outgas phase in a high temperature process, the adhesive is easily absorbed or fixed into a surface of semiconductor device and semiconductor elements due to its low surface energy thereby to contaminate the device and have remarkable adverse effect on semiconductor reliability associated with wire bonding or die attachment. The silicon adhesive is expected to be advantageous in terms of a leak of sealing resin because of its high adhesiveness, but due to the high strain resulting from high viscosity, a flow of sealing resin at a high temperature and high pressure force into between the lead frame and the adhesive layer thereby causing a leak of sealing resin.